Dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate]: An exceptionally effective Rh(II) catalyst for enantiotopically selective aromatic C-H insertions of diazo ketoesters

Article abstract of DOI:10.1016/S0957-4166(03)00075-2

Dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate], Rh₂[(S)-TFPTTL]₄, in which the phthalimido hydrogen atoms of the parent dirhodium(II) complex are substituted by fluorine atoms, dramatically enhances the reactivity and enantioselectivity (up to 97% ee) in intramolecular aromatic C-H insertion reactions of methyl 4-alkyl-2-diazo-4,4-diphenyl-3-oxopropionates. Catalysis with the use of 0.001 mol% of Rh₂[(S)-TFPTTL]₄ has achieved the highest turnover number (up to 98,000 with the methyl substituent) ever recorded for chiral dirhodium(II) complex-catalyzed carbene transformations, without compromising the yield or enantioselectivity of the process.

Full text of DOI:10.1016/S0957-4166(03)00075-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 817–821
Dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate]:
an exceptionally effective Rh(II) catalyst for enantiotopically
selective aromatic C–H insertions of diazo ketoesters
Hideyuki Tsutsui, Yukiko Yamaguchi, Shinji Kitagaki, Seiichi Nakamura, Masahiro Anada and
Shunichi Hashimoto*
Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
Received 10 December 2002; accepted 1 January 2003
Abstract—Dirhodium(II) tetrakis[N-tetrafluorophthaloyl-(S)-tert-leucinate], Rh2[(S)-TFPTTL]4, in which the phthalimido hydro-
gen atoms of the parent dirhodium(II) complex are substituted by fluorine atoms, dramatically enhances the reactivity and
enantioselectivity (up to 97% ee) in intramolecular aromatic C–H insertion reactions of methyl 4-alkyl-2-diazo-4,4-diphenyl-3-oxo-
propionates. Catalysis with the use of 0.001 mol% of Rh2[(S)-TFPTTL]4 has achieved the highest turnover number (up to 98,000
with the methyl substituent) ever recorded for chiral dirhodium(II) complex-catalyzed carbene transformations, without compro-
mising the yield or enantioselectivity of the process. © 2003 Elsevier Science Ltd. All rights reserved.
Transition metal complexes are known to catalyze a
broad spectrum of diazo carbonyl compound transfor-
mations and the advantages of using dirhodium(II)
carboxylate catalysts are well documented: The diazo
decomposition occurs under much milder conditions
and produces higher yields for many processes based on
metal carbene species than are possible with other
catalysts.1 Therefore, it is not surprising that a great
deal of effort continues to be devoted to the design,
synthesis, and evaluation of chiral dirhodium(II) car-
boxylate catalysts, which allows highly enantioselective
carbocyclic and heterocyclic systems to be constructed.2
Doyle pioneered chiral dirhodium(II) carboxamidate
ligands, respectively. McKervey’s Rh2[(S)-BSP]4
exhibits high levels of enantioselectivity with limited
substrates,4 but Davies’ Rh2[(S)-DOSP]4 is an excep-
tionally effective catalyst for a wide variety of enan-
tioselective intermolecular C–H insertions6 and
cyclopropanations,7 a major breakthrough in this field.
Our efforts led to the development of dirhodium(II)
carboxylate catalysts such as Rh2[(S)-PTA]4 3a and
Rh2[(S)-PTTL]4 3d, which incorporate N-phthaloyl-
(S)-amino acids as bridging ligands.8–12 These catalysts
mediate intramolecular C–H insertions of a structurally
diverse array of diazo carbonyl compounds,8 inter-
molecular Si–H insertions,9 intermolecular 1,3-dipolar
cycloadditions via the generation of an ester–carbonyl
ylide,10 and [2,3]-sigmatropic rearrangements via the
intramolecular formation of allylic or propargylic oxo-
nium ylides11 with a maximum of 98, 74, 93 and 79%
ee, respectively. These catalysts contain two phthal-
imido groups in a pair of adjoining ligands, which are
oriented to an axial coordination site of each octahe-
dral rhodium centre. Although the effects of the amino
acid alkyl substituents on the enantioselectivity have yet
to be established, it is believed that the two phthalimido
groups play a pivotal role as enantiocontrollers. In this
respect, by devising dirhodium(II) carboxylate catalysts
such as Rh2[(S)-BPTV]4 4a and Rh2[(S)-BPTTL]4 4b we
have remarkably enhanced the enantioselectivity (typi-
cally 59 to 90% ee) in tandem formation and inter-
molecular 1,3-dipolar cycloaddition of keto-carbonyl
catalysts with
a rigid framework, which include
Rh2[(5S)-MEPY]4 1a, Rh2[(4S)-MEOX]4 1b, and
Rh2[(4S)-MPPIM]4 1c. These catalysts can place a chi-
ral center in close proximity to the rhodium(II) carbene
center,3 but chiral dirhodium(II) carboxylate catalysts
seem ineffective at creating an asymmetric environment
around the rhodium(II) carbene center because of the
conformational flexibility of the carboxylate ligand.
The McKervey4 and Davies5 groups recently developed
Rh2[(S)-BSP]4 2a and Rh2[(S)-DOSP]4 2b, chiral
dirhodium(II) carboxylate catalysts that incorporate N-
benzenesulfonyl derivatives of (S)-proline as bridging
* Corresponding author. Tel.: +81-11-706-3236; fax: +81-11-706-4981;
e-mail: hsmt@pharm.hokudai.ac.jp
0957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0957-4166(03)00075-2

818
H. Tsutsui et al. / Tetrahedron: Asymmetry 14 (2003) 817–821
pounds are thought to proceed via an electrophilic
attack of the rhodium(II) carbene carbon on the aro-
matic ring followed by a 1,2-hydride shift with a con-
current dissociation of the rhodium(II) catalyst and
subsequent aromatization rather than via a direct C–H
insertion mechanism.8b,15,20 Therefore, the electrophilic-
ity of a rhodium(II) carbene carbon is the principal
influence on this process. We previously reported that
Rh2[(S)-PTTL]4 3d mediates aromatic C–H insertion of
3-alkyl-1-diazo-3,3-diphenyl-2-propanones 7 to yield
(S)-1-alkyl-1-phenyl-2-indanones 8 containing a chiral
quaternary carbon atom in up to 98% ee (Eq. (1)).8c
The use of the substituents such as methyl, propyl, and
allyl groups resulted in high enantioselectivities (88–
90% ee), but an exceedingly high order of differentia-
tion of enantiotopic benzene rings was observed with
7b, which bears an ethyl substituent at C-3. These
results provide a strong incentive to enhance the enan-
tioselectivity in a general sense.
ylides. Rh2[(S)-BPTV]4 and Rh2[(S)-BPTTL]4 are char-
acterized by an extension of the phthalimido group
with an additional benzene ring.12 Continuing our work
on improving the catalytic performance by modifying
the sterics and electronics of the phthalimido wall, we
found that using N-tetrafluorophthaloyl-(S)-tert-leuci-
nate as the bridging ligands greatly enhances the cata-
lyst turnover number (up to 98,000) and
enantioselectivity (up to 97% ee) in intramolecular aro-
matic C–H insertion reactions of methyl 4-alkyl-2-
diazo-4,4-diphenyl-3-oxopropionates 6.
(1)
At the onset, the aromatic C–H insertion reactions of
diazo ketoester 6a in dichloromethane at 0°C in the
presence of 2 mol% of Rh2[(S)-PTTL]4 3d, Rh2[(S)-
TFPTTL]4 5a and Rh2[(S)-TCPTTL]4 5b was explored
(Table 1).21 The reactions with the halogenated cata-
lysts 5a and 5b proceeded to completion within 5 min,
which was much faster than expected and resulted in a
high yield of the cyclic ketoester 9a. In contrast, the
parent catalyst 3d required longer reaction times (1 h).
The sense and extent of differentiation between enan-
tiotopic benzene rings were determined by
demethoxycarbonylation of 9a to the known (S)-(−)-1-
methyl-1-phenyl-2-indanone 8a.8b While a uniform
sense of the enantiotopic selection was observed in all
cases, the fluorinated catalyst 5a exhibited the highest
enantioselectivity of 97% ee (entry 2). Catalysts 3d and
5b showed little variation in enantioselectivity (92% ee
versus 91% ee, entry 1 versus 3). These results strongly
suggest that both catalysts 5a and 5b can create an
asymmetric environment similar to 3d. Lowering the
reaction temperature to −10°C had little impact on the
enantioselectivity, but produced a marked decrease in
product yield (entries 4 and 5). Catalysis with 5a was
faster than using 5b. These results also suggest that the
minimal steric influence of the fluorine substituent cou-
pled with its powerful electron-withdrawing effect is
crucial for enhancement of the reactivity and enantiose-
lectivity of this aromatic C–H insertion process. The
utility of Rh2[(S)-TFPTTL]4 was extended to diazo
ketoesters 6b and c, which have ethyl and allyl sub-
stituents at C-3, respectively. Here again, a shorter
reaction time and higher enantioselectivities were
observed in each series (entries 6 versus 7 and 8 versus
9).
It was recently demonstrated that modifying ligands
with electron-donating13 or electron-withdrawing14 sub-
stituents at certain positions could greatly impact the
rate and enantioselectivity of catalytic asymmetric reac-
tions. Based on the general finding that dirhodium(II)
catalysts with strongly electron-withdrawing carboxyl-
ate ligands show remarkably high reactivity towards
diazo carbonyl compounds,15 we envisaged that devel-
oping dirhodium(II) carboxylates that substitute halo-
gen atoms for the phthalimido hydrogen atoms could
facilitate the formation and ensuing transformation of
the intermediate rhodium(II) carbene.16 The enhanced
reactivity should lower the reaction temperature,
thereby leading to higher enantioselectivity. To test the
feasibility of this approach, a new class of dirhodium
(II) carboxylates, Rh2[(S)-TFPTTL]4 5a and Rh2[(S)-
TCPTTL]4 5b17 were prepared from Rh2(OAc)4 by lig-
and exchange reaction18 with N-tetrafluorophthaloyl-
and N-tetrachlorophthaloyl-(S)-tert-leucines, respec-
tively. Their utility was examined through intramolecu-
lar aromatic C–H insertion reactions of methyl
4-alkyl-2-diazo-4,4-diphenyl-3-oxopropionates 6.19 Aro-
matic C–H insertion reactions of diazo carbonyl com-

H. Tsutsui et al. / Tetrahedron: Asymmetry 14 (2003) 817–821
819
Table 1. Enantiotopically selective intramolecular aromatic C–H insertion reaction of diazo ketoester 6 catalyzed by chiral
Rh(II) complexesa
Entry
Substrate
R
Rh(II) complex
Temp. (°C)
Time
Ketoester 9
(S)-2-Indanone 8
[h]D (c, CHCl3)
Yield (%)b
Yield (%)b
Ee (%)
1
2
3
4
5
6
7
8
9
6a
6a
6a
6a
6a
6b
6b
6c
6c
Me
Me
Me
Me
Me
Et
Et
Allyl
Allyl
Rh2[(S)-PTTL]4 3d
0
0
1 h
89
90
87
70
64
98
94
76
69
96
96
97
97
94
94
98
97
94
−52.8 (1.12)
−56.3 (1.26)
−53.8 (1.27)
−57.5 (1.24)
−54.8 (1.30)
−90.2 (1.07)
−92.1 (1.07)
−69.6 (1.25)
−74.1 (1.31)
92c
97c
91c
98c
93c
96d
97d
93e
97e
Rh2[(S)-TFPTTL]4 5a
Rh2[(S)-TCPTTL]4 5b
Rh2[(S)-TFPTTL]4 5a
Rh2[(S)-TCPTTL]4 5b
Rh2[(S)-PTTL]4 3d
Rh2[(S)-TFPTTL]4 5a
Rh2[(S)-PTTL]4 3d
Rh2[(S)-TFPTTL]4 5a
2 min
5 min
20 min
1 h
0
−10
−10
0
2 h
0
0
0
10 min
3 days
9 h
a The following is a representative procedure (entry 2): Rh2[(S)-TFPTTL]4·2EtOAc (10.2 mg, 0.006 mmol, 2 mol%) was added in one portion to
a solution of diazo ketoester 6a (92.5 mg, 0.3 mmol) in CH2Cl2 (1.5 mL) at 0°C. The mixture was concentrated in vacuo and chromatographed
on silica gel to afford 9a (75.4 mg, 90%). A solution of 9a (70 mg, 0.25 mmol) in 90% aqueous DMSO (1.5 mL) was heated at 120°C for 30
min. Standard workup followed by chromatography provided 8a (53.8 mg, 97%).
b Isolated yield.
c Determined by HPLC [column, Daicel Chiralcel OD-H; eluent, 200:1 hexane:iPrOH; retention time, 9.4 min (S, major) and 10.8 min (R, minor)].
d Determined by HPLC [column, Daicel Chiralcel OJ-H; eluent, 9:1 hexane:iPrOH; retention time, 11.3 min (S, major) and 13.0 min (R, minor)].
e Determined by HPLC [column, Daicel Chiralcel OD-H; eluent, 200:1 hexane:iPrOH; retention time, 8.3 min (S, major) and 9.9 min (R, minor)].
Once the effectiveness of Rh2[(S)-TFPTTL]4 was evalu-
ated as a catalyst for this process, the amount of catalyst
required for the reaction of diazo ketoester 6a was
investigated. For optimal results, catalytic asymmetric
reactions based on rhodium(II)–carbene species usually
require between 1–5 mol% of the catalyst. A few examples
effectively employed less than 0.1 mol% of chiral dirhodi-
um(II) catalysts.22 Therefore, we were surprised to find
that the catalyst loading could be dramatically reduced
in this reaction without compromising the yield or the
enantioselectivity (Table 2). When the amount of Rh2[(S)-
PTTL]4 was reduced from the standard 2 mol% to 0.1
mol%, the reaction required 5.5 h more to reach comple-
tion (entries 1 versus 2). When the amount of Rh2[(S)-
TFPTTL]4 was decreased from 2 mol% to 0.01 mol%, the
reaction was completed within 2 h to give, after removal
of the ester group, 2-indanone 8a in a quantitative yield
with essentially the same enantioselectivity (entries 3
versus 4). It is interesting that in each series decreasing
the amount of catalyst increased the yield. Finally, an
exceptionally high order of productivity of Rh2[(S)-
TFPTTL]4 was demonstrated when 0.001 mol% catalyzed
the reaction with the same yield and enantioselectivity as
0.01 mol% of the catalyst (entry 5).23 It is noteworthy that
turnover numbers as high as 98,000 were achieved, which
is the highest ever recorded for chiral dirhodium(II)
complex-catalyzed carbene transformations.
aromatic C–H insertion reactions of methyl 4-alkyl-
2-diazo-4,4-diphenyl-3-oxopropionates. maximum
of 97% ee and turnover numbers as high as 98,000
were achieved. Further studies to explore the scope of
Rh2[(S)-TFPTTL]4-catalyzed reactions are currently in
progress.24
A
Table 2. Enantioselective intramolecular aromatic C–H
insertion reaction of diazo ketoester 6a catalyzed by chiral
Rh(II) complexesa
Entry
Rh(II) complex
(mol%)
Time
Yield (%)b
Ee (%)c
1
Rh2[(S)-PTTL]4
3d (2.0)
Rh2[(S)-PTTL]4
3d (0.1)
Rh2[(S)-TFPTTL]4 2 min 90
5a (2.0)
Rh2[(S)-TFPTTL]4 2 h
5a (0.01)
1 h
89
98
92
93
97
97
97
2
6.5 h
3
4d
5d,e
98
Rh2[(S)-TFPTTL]4 48 h
5a (0.001)
98
a Unless otherwise noted the reactions were conducted using 6a (92.5
mg, 0.3 mmol) in CH2Cl2 (1.5 mL) at 0°C.
b Isolated yield of 9a.
c Determined by HPLC (Daicel Chiralcel OD-H) after demethoxy-
carbonylation.
d Rh2[(S)-TFPTTL]4 (3.0×10−4 M in CH2Cl2) was used.
e The reaction was conducted using 6a (925 mg, 3.0 mmol) in CH2Cl2
(15 mL).
In summary, we demonstrated that Rh2[(S)-TFPTTL]4,
the fluorinated analogue of Rh2[(S)-PTTL]4, is an excep-
tionally effective catalyst for asymmetric intramolecular

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H. Tsutsui et al. / Tetrahedron: Asymmetry 14 (2003) 817–821
L. Org. Lett. 2000, 2, 823; (c) Davies, H. M. L.;
Acknowledgements
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Hashimoto, S. Tetrahedron: Asymmetry 2000, 11, 3855.
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This research was partially supported by a Grant-in-
Aid for Scientific Research on Priority Areas (A)
‘Exploitation of Multi-Element Cyclic Molecules’ from
the Ministry of Education, Culture, Sports, Science and
Technology, Japan. We thank S. Oka, M. Kiuchi, A.
Maeda, and H. Matsumoto of the Center for Instru-
mental Analysis at Hokkaido University for mass mea-
surements and elemental analysis.
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H. Tsutsui et al. / Tetrahedron: Asymmetry 14 (2003) 817–821
821
17. Representative procedure for preparation of new chiral
Rh(II) complexes: Preparation of dirhodium(II) tetra-
kis[N-tetrafluorophthaloyl-(S)-tert-leucinate] 5a: The fol-
lowing procedure is similar to that reported by Callot.18
A mixture of Rh2(OAc)4·2MeOH (253 mg, 0.50 mmol)
and N-tetrafluorophthaloyl-(S)-tert-leucine (920 mg, 2.51
mmol) in chlorobenzene (40 mL) was heated at reflux
with vigorous stirring, while the solvent was distilled off
at a rate such that 5 mL of the solvent was removed per
hour. After completion of the reaction (3 h) was confirm-
ed by TLC analysis, the dark green mixture was cooled to
rt and diluted with EtOAc (40 mL). The whole was
successively washed with saturated aq. NaHCO3 (3×10
mL), water (10 mL) and brine (2×10 mL), and dried over
Na2SO4. Filtration and evaporation in vacuo gave crude
product (1.1 g of green semisolid) which was chro-
matographed on silica gel (2:1 hexane/EtOAc) followed
by recrystallization (6:1 hexane/EtOAc, 20 mL) to
provide bis(ethyl acetate) adduct of 5a as green needles
(770 mg, 83%): TLC Rf 0.69 (2:1 hexane/EtOAc); mp
>280°C (hexane/EtOAc); [h]2D2 +291.6 (0.049, C6H6); IR
Cl, 28.73, found: C, 38.71; H, 2.66; N, 2.96; Cl, 28.93%.
18. Callot, H. J.; Metz, F. Tetrahedron 1985, 41, 4495.
19. Methyl 4-alkyl-2-diazo-4,4-diphenyl-3-oxopropionates 6
were prepared from diphenylacetic acid by the following
sequence: (1) BuLi, THF, 0°C, then RX (MeI, EtI and
CH2ꢀCHCH2Br for 6a,b and c, respectively), 0°C; (2) cat.
DMF, SOCl2, toluene, reflux; (3) LiCH2CO2Me, THF,
−78°C; (4) MsN3, Et3N, MeCN.
20. (a) Doyle, M. P.; Shanklin, M. S.; Pho, H. Q.; Mahapa-
tro, S. N. J. Org. Chem. 1988, 53, 1017; (b) Cox, G. G.;
Moody, C. J.; Austin, D. J.; Padwa, A. Tetrahedron 1993,
49, 5109.
21. We reported that in the presence of 2 mol% of Rh2[(S)-
PTTL]4 aromatic C–H insertion of diazo ketoester 6a
resulted in a similar product yield and asymmetric induc-
tion as diazo ketone 7a.8c Diazo ketoester 6 was chosen
as the substrate for this study due to its stability and ease
of handling.
22. (a) Doyle, M. P.; Kalinin, A. V. J. Org. Chem. 1996, 61,
2179; (b) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.;
Kong, N.; Fall, M. J. J. Am. Chem. Soc. 1996, 118, 6897;
(c) Doyle, M. P.; Austin, R. E.; Bailey, A. S.; Dwyer, M.
P.; Dyatkin, A. B.; Kalinin, A. V.; Kwan, M. M. Y.;
Liras, S.; Oalmann, C. J.; Pieters, R. J.; Protopopova, M.
N.; Raab, C. E.; Roos, G. H. P.; Zhou, Q.-L.; Martin, S.
F. J. Am. Chem. Soc. 1995, 117, 5763.
23. A representative procedure (Table 2, entry 5): Rh2[(S)-
TFPTTL]4·2EtOAc (3.0×10−4 M in CH2Cl2, 0.1 mL,
0.001 mol%) was added to a solution of diazo ketoester
6a (925 mg, 3.0 mmol) in CH2Cl2 (15 mL) at 0°C. After
stirring for 48 h at 0°C, the mixture was concentrated in
vacuo and purified by column chromatography (silica gel,
15:1 hexane/EtOAc) to give 9a (824 mg, 98%) as a pale
blue oil. A solution of 9a (810 mg, 2.89 mmol) in 90%
aqueous DMSO (5 mL) was heated at 120°C for 30 min.
After the mixture cooled to rt, it was partitioned between
EtOAc (50 mL) and water (20 mL). Then the organic
layer was washed with brine, and dried over Na2SO4.
Filtration and evaporation, followed by column chro-
matography (silica gel, 15: 1 hexane/EtOAc) afforded 8a
(623 mg, 97%) as a pale orange oil; [h]2D5 −58.3 (c1.33,
CHCl3) [lit.,8b [h]D25 −59.7 (c1.03, CHCl3) for (S)-8a].
24. Very recently, we found that Rh2[(S)-TCPTTL]4 is an
effective catalyst for enantioselective benzylic amidation,
see: Yamawaki, M.; Tsutsui, H.; Kitagaki, S.; Anada, M.;
Hashimoto, S. Tetrahedron Lett. 2002, 43, 9561.
1
(KBr) 3503, 2971, 1788, 1730, 1612, 1514, 1406 cm−1; H
NMR (270 MHz, CDCl3) l 1.08 (36H, s), 1.26 (6H, t,
J=7.3 Hz), 2.05 (6H, s), 4.13 (4H, q, J=7.3 Hz), 4.69
(4H, s); 13C NMR (67.8 MHz, CDCl3) l 14.2, 21.1, 27.7,
35.8, 60.8, 61.7, 113.5 (2J=7 Hz), 143.2 (dmulti, J:270
Hz), 144.9 (dmulti, J:264 Hz), 161.9, 163.6, 172.2,
186.4; LRMS (FAB) m/z 1535 (M++H). Anal. calcd for
C56H40F16N4O16Rh2·2EtOAc·H2O: C, 44.46; H, 3.38; N,
3.24; F, 17.58. Found: C, 44.25; H, 3.52; N, 3.29; F,
17.77%. Dirhodium(II) tetrakis[N-tetrachlorophthaloyl-
(S)-tert-leucinate],
5b:
Prepared
starting
from
Rh2(OAc)4·2MeOH (506 mg, 1.0 mmol) and N-tetra-
chlorophthaloyl-(S)-tert-leucine (2.0 g, 5.0 mmol) by the
same procedure as above. Column chromatography (sil-
ica gel, 15:1 benzene/Et2O) followed by recrystallization
(1:1 hexane/EtOAc, 40 mL) provided bis(ethyl acetate)
adduct of 5b as green prisms (1.36 g, 69%): TLC Rf 0.36
(5:1 hexane/EtOAc); mp >280°C (hexane/EtOAc); [h]D22
+328.6 (0.054, C6H6); IR (KBr) 3650, 3495, 2965, 1784,
1
1728, 1613, 1385, 1372 cm−1; H NMR (270 MHz, C6D6,
60°C) l 0.72 (6H, t, J=7.0 Hz), 1.43 (36H, s), 1.62 (6H,
s), 3.65, (4H, q, J=7.0 Hz), 5.24 (4H, s); 13C NMR (67.8
MHz, C6D6) l 13.7, 20.4, 28.3, 36.9, 60.8, 61.7, 127.3,
127.8, 129.2, 129.9, 139.4, 139.8, 163.0, 163.6, 171.9,
186.8; LRMS (FAB) m/z 1798 (M++H). Anal. calcd for
C56H40Cl16N4O16Rh2·2EtOAc: C, 38.94; H, 2.86; N, 2.84;